1. Field of the Invention
This invention relates to a crash sensor suitable for the detection of crash of a vehicle with a triggering system of a passenger protecting apparatus calld an air bag.
2. Description of the Prior Art
Conventionally, crash sensors suitable for the detection of crash of a vehicle with a triggering system of a passenger protecting apparatus calld an air bag are divided into two types including a first type which employs a sensing mass and a second type which employs an electronic accelerometer. One of such crash sensors as employs an electronic accelerometer is disclosed in U.S. Pat. No. 3,701,903. This crash sensor comprises means for time integrating an acceleration, when the time integrated value exceeds a predetermined value, a passenger protecting apparatus calld an air bag is triggered. And another modified crash sensor is disclosed in U.S. Pat. No. 3,911,391, wherein, before time integration is performed, a predetermined acceleration is subtracted from an acceleration so that the passenger protecting apparatus can not be triggered, when an acceleration of a vehicle riding on a rough road is not such as to hurt a passenger.
With the crash sensor, however, the passenger protecting apparatus cannot always be triggered within a required period of time in all forms of forecasted crash with an actual vehicle. Particularly, acceleration waveform upon a low speed head-on crash in which triggering the passenger protecting apparatus is not required and upon a high speed oblique or pole crash of a vehicle which has a normal rigid body in which triggering the passenger protecting apparatus is required are very similar to each other and cannot be identified from each other at an initial stage of crash until the passenger protecting apparatus is triggered. In particular, as seen in FIG. 4, an acceleration waveform (shown by a dashed line) for a low speed head-on crash in which triggering the passenger protecting apparatus is not required, and another acceleration waveform (shown by a thick solid line for a high speed oblique or pole crash in which triggering the passenger protecting apparatus is required are very similar to each other over the period of time after the crash that is indicated from time 1 to time 2, as shown in FIG. 4. It is over this period of time that it must be determined whether the passenger protecting apparatus should be triggered. Accordingly, as shown in FIG. 5, it is difficult to identify them from each other even with a crash sensor which subtracts a predetermined acceleration from a measured acceleration and then time integrates the difference (V1') thus obtained. As a result, if priority is given to a triggering requirement, unnecessary triggering of the apparatus will take place upon a low speed head-on crash. On the contrary, if priority is given to a no triggering requirement, a delay or lack of triggering of the apparatus will take place upon a high speed oblique or pole crash.
In order to solving this problem, this applicant has shown another invention of a crash sensor in JP 74457/90. According to one aspect of the JP 74457/90 crash sensor, there is provided a crash sensor which detects a crash of a vehicle from an acceleration waveform of an accelerometer to trigger the passenger protecting apparatus calld an air bag, comprising means for peak cutting any portion of the input acceleration waveform lower than a predetermined value, means for time integrating the value obtained by the said peak cutting, means for subtracting a time integrated value of a predetermined function from the value obtained by the said time integration, and means for first comparing the value obtained by the said subtraction with a predetermined time function value to develop a triggering signal.
According to the crash senser, while acceleration waveforms upon a low speed head-on crash and upon a high speed oblique or pole crash are almost the same in average acceleration at an initial stage of a crash which coincides with a triggering request time, an acceleration waveform upon a high speed oblique or pole crash has considerable vibration components as seen in FIG. 4 due to buckling, vibrations and so forth of a car body. Meanwhile, since most part of impact energy upon a low speed head-on crash is absorbed by an energy absorbing apparatus such as a bumper, vibration components are not so considerable. The crash sensor has been made with perceiving such difference in characteristic between the two acceleration waveforms. Accordingly, as seen in FIG. 5, such time integrated value of an acceleration upon a high speed oblique or pole crash which involves much vibration components is greater than a time integrated value of a mere difference of an acceleration from a fixed acceleration. Consequently, an identification can be made between a low speed head-on crash and a high speed oblique or pole crash with a higher degree of accuracy.
Forms of actual crash, however, are very complicated. In case of a high speed oblique or pole crash of a vehicle which has a less rigid body, and in case of a compound crash which is a compound of various forms of crash (e.g. a vehicle firstly crashed into a fence, secondly crashed into walls and continuously crashed into something.), an acceleration waveform (shown by a thin solid line curve) is low at an initial stage of a crash which coincides with a triggering request time as given as a section from 1 to 2 in FIG. 4. Namely, in these cases, the passenger protecting apparatus cannot be triggered within a triggering request time. In these cases, the means for a first comparing, as seen in FIG. 5, solves only the problem of an identification between a low speed head-on crash and a high speed oblique or pole crash of a vehicle which has a normal rigid body. Thus, it is still possible for a delay or lack of triggering of the apparatus to take place.